Method of forming miniature electrical conductors

ABSTRACT

A PROCESS FOR THE MANUFACTURING OF MINIATURE ELECTRICAL CONDUCTORS WHICH COMPRISES ETCHING A BASE MATERIAL TO A PREDETERMINED SHAPE FOLLOWED BY ELECTROPLATING MATERIAL ONTO THE BASE MATERIAL UNTIL THE DESIRED CONDUCTOR CONFIGURATION IS OBTAINED.

Dec. 4, 1973 E. c. JOHNSON ETAL 3,776,820

METHOD OF FORMING MINIATURE ELECTRICAL CONDUCTORb Fig. 2

United States Patent O US. Cl. 204-45 3 Claims ABSTRACT OF THEDISCLOSURE A process for the manufacturing of miniature electricalconductors which comprises etching a base material to a predeterminedshape followed by electroplating material onto the base material untilthe desired conductor configuration is obtained.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates generally to miniature conductors and, more specifically, to aprocess for forming miniature electrical conductors having squarecorners.

Description of the prior art Generally conductors having a thinrectangular or square cross section are utilized in electrostaticprinting machines or miniature electrical circuits. In thoseapplications one wishes to achieve contact with as large a portion ofthe conductor surface as possible by having conductors with squarecorners. In the past it has been difficult if not impossible to makeminiature electrical conductors by etching because the etching processproduced conductors having a tapered configuration (known in the tradeas etch factor). Similarly it has been diflicult if not impossible tomake miniature electrical conductors by electroplating because theelectroplating produces conductors with ridges on the corners. Thus, ithas been ditficult if not impossible to obtain a conductor having asquare cross section or a rectangular cross section using conventionalelectroplating or etching techniques. The present invention comprisesthe discovery of a process for producing a substantially square orrectangular cross section conductor through the utilization of bothetching and electroplating techniques.

SUMMARY OF THE INVENTION Briefly, the present invention comprisesetching a conductor material to a predetermined shape such as anisosceles trapezoid and then electroforming material onto the isoscelestrapezoid until one produces a conductor having square corners.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a cross-sectional view ofa material which has been etched away to form a base for electroplatingon; and

FIG. 2 shows a cross-sectional view of the base with materialelectroplated thereon.

DESCRIPTION OF THE PREFERRED PROCESS In the preferred process of thisinvention, one places a metal conductor such as copper on a dielectricmaterial or support surface. The support surface may be any typical typeof dielectric. Typical of such dielectric support surfaces are polyimidefilms, however, no limitation is intended thereto. Referring to FIG. 1,we have designated certain reference characters and reference letters toassist in describing the process and relationships of the variousdimensions of the article to be formed into a conductor.

FIG. 1 shows a dielectric material 11 having a conductor 13 with aresist material 12 located on top of conductor 13. In this figureconductor 13 has been etched to its predetermined base shape which is anisosceles trapezoid. While FIG. 1 shows conductor 13 with straight welldefined sides, in actual practice the etched sections are slightlycurved and have somewhat of an irregular surface.

The characteristic dimensions denoted in FIG. 1 are as follows: wrepresents the width of resist material 12 on conductor 13, W,represents the width of the top of the isosceles trapezoid formed byconductor 13, w; represents the width of the base of the isoscelestrapezoid formed by conductor 13 and h; represents the height of theisosceles trapezoid. The most important dimension of this process arethe angles designated by Typically, should be maintained on the order ofabout 10 to 30 in order to obtain a conductor having the shape shown inFIG. 2.

FIG. 2 shows the final electroformed conductor 14 with the squarecorners. In this particular embodiment, conductor 14 has a square crosssection, however, the cross section could also be rectangular. The widthof conductor 14 is designated by W2 and the height of conductor 14 isdesignated by k To illustrate the process of the invention, referenceshould be made to the following typical examples. Although in practicemultiple conductors are simultaneously etched and then electroplated,for purposes of illustrating the process of the invention only oneconductor will be described.

EXAMPLE 1 In order to obtain a conductor with square cross section, acopper sheet of thickness il of 1.4 mils was placed on dielectric 11.The copper was selectively covered with a resist having a width w of 2mils. The copper was then etched until the dimension W2 wasapproximately 1 mil and the dimension W1 was about 1.5 mils. With athickness h of 1.4 mils, one obtains an angle on the order of 10. Withthese dimensions base conductor 13 has the shape of an isoscelestrapezoid. Next, resist material 12 was removed and copper waselectroplated on top of base conductor 13 until the dimension W2 and I1were approximately 5 mils. When conductor 14 reached this size thecorners of the conductors were substantially square.

EXAMPLE 2 In this example all the dimensions were identical to Example 1with the exception that the thickness h was .7 mil rather than 1.4 mils.This produced an angle 5 which is approximately 20. With this angle theelectroplating produced a conductor with a substantially square crosssection and a final dimension of approximately 5 mils by 5 mils.

EXAMPLE 3 In another example, the dimension w Was 2 mils, dimension W1was 3 mils, the height h, was 1 mil and the width of the resist w was 4mils. In this particular process the angle was approximately 27 and thefinal dimensions W2 and h were on the order of about 5 mils.

In all of the above examples the initial dimensions were such that afinal configuration turned out to be approximately square in crosssection, however, lengthening the dimension w with respect to its heightproduces a conductor that is rectangular rather than square.

The particular reason that our process produces a. conductor with squarecorners is through the combination of the two techniques of etching andelectroplating. That is, the etching process undercuts resist material12 to form a sloping side making angle which is indicated in FIG. 1.This is a natural occurring process if the article is etched in anetching bath without any compensation techniques. It is thisundercutting phenomena which prevents one from etching small conductorswith square corners. On larger conductors which are over 15 mils thickone can oftentimes compensate for angled sides, however, it is difficultif not impossible to do where there are multiple conductors to be formedwhich are typically less than mils in thickness. Thus, etching producesa base configuration with substantially the shape of an isoscelestrapezoid.

In electroplating a naturally occurring phenomena is build-up ofelectroplated material on the corners. This build-up on the corners ifused with a square shaped base member or cathode, will produce an offsetor enlarged corner on the cathode. Thus, one cannot electroplate smallconductors with square cross sections because of the naturally occurringphenomena of build-up on the corners. However, because the corners ofour base conductor 13 have been undercut or beveled due to the etching,the electroplating technique and etching techniques compensate for eachother to produce a conductor with square corners. That is, theelectroplating techniques produce a buildup on the beveled corners thatresults in a conductor with square corners.

While many examples have been tried in producing a particular squarecorner on the conductor, the suitable ranges of the angle have rangedfrom approximately 10 to 30 and the ratio w /w has been found to rangefrom approximately 3 to 4 and the ratio h /h has ranged fromapproximately 2.8 to 8.0. However, the ratio of the dimensions are notsignificant, only typical to obtaining a conductor with a square crosssection. The important and critical dimension is the maintaining of theangle in the range of 10 to 30 to produce a conductor which has squarecorners.

4 We claim: 1. In the production of printed circuits by chemicalmilling, a process for making the electrical conductor lines havingsquare corners comprising the steps of:

(a) first etching the electrical circuit pattern from a layer ofelectrically conductive material attached to a layer of insulatingmaterial; then (b) continuing the etching to reduce the thickness andwidth of the conductors to dimensions less than the desired finaldimensions; and then (0) electroplating material to build up the widthand thickness of the conductors to the final dimensions.

2. The process of claim 1 wherein the step of etching the electricalcircuit pattern is continued until an isosceles trapezoid having sidesthat make an angle 43 with a line perpendicular to a base of theisosceles trapezoid, said angle ranging from about 10 to about 30.

3. The process of claim 2 including the step of applying a layer ofetchant resist on top of the electrical circuit pattern.

References Cited UNITED STATES PATENTS 8/1968 Nordin 174-68.5 X

OTHER REFERENCES DARRELL L. CLAY, Primary Examiner US. Cl. X.R.

